Mesh fabrics made of natural fibers such as silk, or inorganic fibers such as stainless wires have been widely used as printing screen mesh cloths. In recent years, however, mesh cloths made of organic fibers such as nylon or polyester fibers have been frequently used, because of their flexibility, durability and dimensional stability. In particular, at present, screen mesh cloths made of polyester monofilaments have been widely used, because they are less affected by water than nylon screen mesh cloths, and relatively inexpensive.
In the current field of electronic circuit printing for home appliances, cellular phones, personal computers, etc., however, the need for an improvement in printing precision becomes demanding. Therefore, there has been a demand for screen mesh cloths having a finer mesh and excellent in dimensional stability that elongation is small during cloth tensioning or the like. Thus, there has been a demand for raw yarns for screen mesh cloths having fine fineness, high strength, and high modulus.
In general, it is known that in order to form high-strength, high modulus polyester fibers, the process of forming raw fibers should include performing high-ratio drawing so that high degrees of orientation and crystallization can be obtained. However, if high-ratio drawing is performed, mechanical strain, namely, stress is generated and accumulated in the fiber due to a sudden structural change. The mechanical strain tends to decrease with time, which is called stress relaxation. When the fiber obtained by high-ratio drawing is wound on a pirn, the stress relaxation often does not uniformly proceed over the pirn package, so that the part where the stress relaxation does not proceed develops gloss anomalies. Such anomalies are called pirn barre.
At present, a screen mesh cloth after weaving is used in printing, after a process including applying an emulsion thereto and subjecting it to exposure and curing so that an electronic circuit can be transferred thereto. Therefore, if halation of the applied light occurs in the process of exposing the emulsion to light and curing the emulsion, printing precision will be reduced. In order to prevent the reduction in printing precision, the fabric is dyed with a light color dye after the weaving so that halation can be reduced. However, the pirn barre portion remains as an abnormal stripe portion even after the dyeing and therefore may reduce the screen mesh cloth quality and cause a gloss difference from the normal portion, which may cause exposure unevenness during the exposure of the emulsion to light. As a result, the quality of the screen mesh cloth, which is reduced in printing precision, is not suitable for high-mesh and high-precision printing.
Snarl associated with polyester monofilaments also causes a problem against the production of high-quality screen mesh cloths for use in precise printing. Generally, in a process of weaving a screen mesh cloth, a batch of about 600 to 800 warp yarns are wound on a warping drum at an unraveling speed of 200 m/minute to 500 m/minute in a partial warping machine. In this warping process, if over unraveling occurs due to temporary stop of operation or the like, “fiber slack” will occur, and filaments will be entangled with one another, which will fixed in the form of a twist yarn. This trouble is so-called snarl. If the operation is started again, the snarl with maintaining its shape will be entangled in the warping drum, which will cause warp breakage during weaving, and the snarl will be partially woven into a mesh cloth, so that the quality of the cloth is significantly reduced. When fine-size polyester monofilaments such as those with a fineness of 13 dtex or less are used, snarl will get worse.
A known conventional method for producing a polyester monofilament includes performing spinning once, winding an undrawn fiber, then subjecting the undrawn fiber to one-step or multi-step drawing at a speed of 500 to 1,500 m/minute using a known drawing machine (draw twister), and winding the drawn fiber into a pirn shape. In a draw twister, however, the winding tension increases due to the friction against travelers, so that the degree of relaxation of the residual shrinkage stress on the yarn is different between the end and the center of the package, which makes it impossible to avoid pirn barre (crosswise stripes with a gloss difference, which are periodically formed in the crosswise direction). Since the fiber is twisted by the drawing machine (draw twister), the problem of snarl also occurs.
There is also a known method including drawing an undrawn fiber using a known drawing machine (draw twister) and winding the drawn fiber into a pirn shape, wherein the pirn package is shaped in such a manner that the ratio of its end area is made as small as possible, so that the difference in residual shrinkage stress on yarn between the end and the center of the package can be reduced and that pirn barre can be avoided. Unfortunately, this method, which is substantially one-step drawing, cannot produce a high-strength and high-modulus polyester monofilament. In addition, since the fiber is twisted by the drawing machine (draw twister), the problem of snarl also occurs.
A known method for producing a polyester monofilament includes a so-called direct spinning-drawing method, in which a spun undrawn fiber is directly subjected drawing and winding without being temporarily wound. There has been proposed a method that includes operating, at a speed of 3,000 m/minute or more, a stretching system including a tension applying roll, a heated supply roll, a heated stretching roll, and a non-heated godet roll; and stretching the yarn by 0.1% to 10% between the heated stretching roll and the non-heated godet roll (Patent Literature 1). There is also proposed another method in which pirn winding is performed after direct spinning-drawing is performed by the same method (Patent Literature 2). Unfortunately, these methods, which are both substantially one-step drawing, cannot produce a high-strength and high-modulus polyester monofilament as desired according to the present disclosure. In addition, these methods cannot achieve both high modulus and uniform relaxation of stress, namely, prevention of pirn barre, as desired according to the present disclosure.
There is proposed a method of producing a polyester monofilament by direct spinning-drawing, which includes extruding a polyester monofilament from a spinneret, solidifying it by cooling, then applying a finishing agent (oil agent) to the polyester monofilament yarn, feeding the yarn at 300-800 m/minute, and then subjecting the yarn to multi-step drawing without winding the undrawn yarn temporarily, after the yarn is allowed to pass through three or more hot rolls sequentially (Patent Literature 3). Unfortunately, this method has a problem in which the production of a high-modulus monofilament with a fine fineness such as a fineness of 13 dtex or less suffers from fiber falling during winding, namely, a reduction in yarn-making ability, deterioration of package, and unraveling failure. This method cannot achieve both high modulus and uniform relaxation of stress, namely, prevention of pirn barre, as desired according to the present disclosure.
As described above, conventional techniques have not been able to achieve conflicting goals such as the production of high-strength and high-modulus raw fibers and the prevention of pirn barre.
Therefore, there has been a strong demand for a polyester monofilament that has characteristics necessary for the production of screen mesh cloths for use in precise printing, such as fine fineness, high strength, and high modulus, provides excellent dimensional stability when used to form screen mesh cloths, is free from such a problem as pirn barre or snarl, and provides excellent mesh cloth quality.